It is hard to tell without a picture of the legs. Is there swelling of the whole leg or the joints? Is the leg harder or softer than normal? Look very closely in case there might be a plastic band or a string around the ankle that could be cutting off circulation. This is a common problem when people get chickens, and don't realize they have been banded.

She could have a slipped tendon. That is very hard to treat even when it is first discovered. The hock joint is usually swollen and the tendon can eventually rupture. Most small flock owners try to give a little more attention and keep chicks with leg problems in smaller areas near food and water. They can be picked on in larger groups. There is a good thread called "Slipped tendon--has anyone tried to fix this," that you can read if you go to the top of this page to the search box and enter that.